Display device and method for manufacturing same

ABSTRACT

A display device including a substrate having thin film transistors (TFT) comprising: the TFT including an oxide semiconductor film, a gate electrode and an insulating film formed between the oxide semiconductor film and the gate electrode, wherein a first aluminum oxide film and a second aluminum oxide film, which is formed on the first aluminum oxide film, are formed between the insulating film and the gate electrode, an oxygen concentration in the first aluminum oxide film is bigger than an oxygen concentration in the second aluminum oxide film.

CLAIM OF PRIORITY

The present application is a continuation application of InternationalApplication No. PCT/JP2013/028405, filed on Jul. 30, 2018, which claimspriority to Japanese Patent Application No. 2017-207026, filed on Oct.26, 2017, the contents of these applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION (1) Field of the Invention

The present invention relates to the display device using oxidesemiconductor TFTs, wherein increase of leak current due to decrease ofresistance in the channel caused by reduction of the oxide semiconductoris suppressed.

(2) Description of the Related Art

A liquid crystal display device has a TFT substrate, a counter substrateopposing to the TFT substrate, and a liquid crystal layer sandwichedbetween the TFT substrate and the counter substrate. The TFT substratehas plural pixels arranged in matrix; each of the pixels has a pixelelectrode and a Thin Film Transistor (TFT). A transmittance of light ineach of the pixels is controlled by liquid crystal molecules; thus,images are formed. On the other hand, the organic EL display devicedisplays color images by arranging pixels, which emit light, in matrix;each of the pixels has an organic EL display layer, TFTs, and others.The organic EL display device does not need the backlight; thus it ismore suitable for a flexible display device.

In the display device, the TFTs are used as switching elements in thepixels, or for the peripheral driving circuits, etc. Since the TFT ofthe oxide semiconductor has high OFF resistance, it is suitable for aswitching transistor. In addition, the TFT of the oxide semiconductorhas a merit that it can be manufactured by lower process temperaturethan that of the TFT of the a-Si (amorphous silicon).

In the display device, several insulating films are used as interlayerinsulating films. Even a silicon oxide (SiO) film and a silicon nitride(SiN) film are mainly used for the insulating films, an aluminum oxidefilm is occasionally used for an interlayer insulating film. The patentdocument 1 discloses: forming the gate electrode by aluminum; formingthe aluminum oxide on the surface of the aluminum by anode oxidization;thus, improving adhesive strength between the gate electrode and theresist. The patent document 1 also discloses the aluminum oxide in thethrough hole is eliminated by etching after the through hole is formed.

PRIOR ART DOCUMENTS Patent Document 1:

-   Japanese patent application laid open No. Hei 9-213968

SUMMARY OF THE INVENTION

In the TFT of the oxide semiconductor, leak current of the TFT increaseswhen the resistance of the channel decreases due to the reduction of theoxide semiconductor in the channel. One reason of the reduction of theoxide semiconductor is that oxygen is extracted from the oxidesemiconductor by metal. The gate electrode made of metal is set abovethe oxide semiconductor via an insulating film. Therefore, there is arisk that the oxygen is extracted by the gate electrode from the oxidesemiconductor through the insulating film.

In the meantime, the oxygen rich SiO film contacts the channel of theoxide semiconductor film and supplies oxygen to the oxide semiconductor.When concentration of the oxygen in the SiO film decreases, however,supply of the oxygen to the oxide semiconductor decreases; consequently,the oxide semiconductor tends to be reduced easily.

The purpose of the present invention is to suppress the phenomenon thatoxygen is extracted from the oxide semiconductor film; thus, to realizethe TFT, in which leak current is kept low.

The present invention overcomes the above explained problem; theconcrete structures are as follows.

(1) A display device including a substrate having thin film transistors(TFT) comprising:

the TFT including an oxide semiconductor film, a gate electrode and aninsulating film formed between the oxide semiconductor film and the gateelectrode,

wherein a first aluminum oxide film and a second aluminum oxide film,which is formed on the first aluminum oxide film, are formed between theinsulating film and the gate electrode,

an oxygen concentration in the first aluminum oxide film is bigger thanan oxygen concentration in the second aluminum oxide film.

(2) A display device including a substrate having thin film transistors(TFT) comprising:

the TFT including an oxide semiconductor film, a gate electrode and aninsulating film formed between the oxide semiconductor film and the gateelectrode,

wherein an aluminum oxide film is formed between the insulating film andthe gate electrode,

an oxygen concentration in the aluminum oxide film is bigger at a sideof the insulating film than an oxygen concentration at a side of thegate electrode.

(3) A manufacturing method of a display device comprising:

the display device including a substrate in which the thin filmtransistors (TFT) are formed,

the TFT including an oxide semiconductor film, a gate electrode and aninsulating film formed between the oxide semiconductor film and the gateelectrode,

wherein

forming a first aluminum oxide film on the insulating film,

forming a second aluminum oxide film on the first aluminum oxide film,an oxygen concentration of the second aluminum oxide film is smallerthan an oxygen concentration of the first aluminum oxide film,

forming a metal for the gate electrode, and

patterning the gate metal, the second aluminum oxide film and the firstaluminum oxide film.

(4) A manufacturing method of a display device comprising:

the display device including a substrate in which the thin filmtransistors (TFT) are formed,

the TFT including an oxide semiconductor film, a gate electrode and aninsulating film formed between the oxide semiconductor film and the gateelectrode,

wherein

forming a first aluminum oxide film on the insulating film,

forming a gate electrode,

patterning the gate electrode,

forming a second aluminum oxide film between the gate electrode and thefirst aluminum oxide film by annealing the gate electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the liquid crystal display device;

FIG. 2 is a plan view of the pixel in the display area of the liquidcrystal display device;

FIG. 3 is a cross sectional view of the liquid crystal display device;

FIG. 4 is a detailed cross sectional view of the TFT and its vicinity;

FIG. 5 is an enlarged cross sectional view of the gate electrode and itsvicinity;

FIG. 6 is a cross sectional view of the sputtering equipment;

FIG. 7 is a graph that shows sputtering modes;

FIG. 8 is a cross sectional view that the first aluminum oxide film isformed on the insulating film;

FIG. 9 is a cross sectional view that the second aluminum oxide film isformed on the first aluminum oxide film;

FIG. 10 is a cross sectional view that the metal for the gate electrodeis formed on the second aluminum oxide film;

FIG. 11 is a cross sectional view that the gate electrode, the secondaluminum oxide film and the first aluminum oxide film are patterned;

FIG. 12 is a cross sectional view that ion implantation is beingconducted;

FIG. 13 is a cross sectional view of second embodiment;

FIG. 14 is a cross sectional view of third embodiment;

FIG. 15 is a cross sectional view that the first aluminum oxide film isformed on the insulating film;

FIG. 16 is a cross sectional view that the metal for the gate electrodeis formed on the first aluminum oxide film;

FIG. 17 is a cross sectional view that the gate electrode, the firstaluminum oxide film and the insulating film are patterned;

FIG. 18 is a cross sectional view that annealing is conducted after theinterlayer insulating film is formed over the gate electrode and theoxide semiconductor;

FIG. 19 is a graph that shows the relation between the temperature andoxygen desorption from the aluminum oxide film;

FIG. 20 is a cross sectional view that shows a problem of electricalconnection to the gate electrode according to third embodiment;

FIG. 21 is a graph that shows a way to solve the problem of electricalconnection of the gate electrode in the through hole according to thirdembodiment;

FIG. 22 is a cross sectional view of the liquid crystal display deviceaccording to fourth embodiment; and

FIG. 23 is a cross sectional view of the liquid crystal display devicethat shows feature of fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail referring to thefollowing embodiments. In the explanation below, the invention is mainlyexplained by examples of the liquid crystal display device; however, thepresent invention is applicable to the organic EL display device andother display devices, too.

First Embodiment

FIG. 1 is a plan view of a liquid crystal display device, which thepresent invention is applied. In FIG. 1, the TFT substrate 10 in whichTFTs and pixel electrodes are formed and the counter substrate 200adhere to each other at their periphery by the sealing material 150; theliquid crystal is sandwiched between the TFT substrate 10 and thecounter substrate 200. The area surrounded by the sealing material 150is the display area 500. The peripheral driving circuits 600 are formedat both sides of the display area 500; a part of the peripheral drivingcircuits 600 overlaps with the sealing material 150 in a plan view.

In FIG. 1, in the display area 500, the scanning lines 1 extend in thelateral direction (x direction) from the peripheral driving circuits 600and are arranged in the longitudinal direction (y direction). The videosignal lines 2 extend in the longitudinal direction and are arranged inthe lateral direction. Video signals are supplied to the video signallines 2 from driver IC 160, which is installed in the terminal area 170.The pixel 3 is formed in the area surrounded by the scanning lines 1 andthe video signal lines 2.

In FIG. 1, the TFT substrate 10 is made bigger than the countersubstrate 200; the area of the TFT substrate 10 that does not overlapwith the counter substrate 200 is the terminal area 170 where the driverIC 160 is installed. In the meantime, the flexible wiring substrate isconnected to the terminal area 170 to supply power and signals to theliquid crystal display device.

The TFTs of the oxide semiconductor are used in the liquid crystaldisplay device of FIG. 1. The TFT of the oxide semiconductor has afeature of low leak current; therefore, it is suitable for the switchingelement in the pixel in the display area. On the other hand, the TFT ofthe poly silicon semiconductor has a high leak current, however, themobility of carriers is high; thus, it is sometimes used for the drivingTFTs in the peripheral driving circuit.

FIG. 2 is a plan view of the pixel in the display area 500 of the TFTsubstrate 10. In FIG. 2, the scanning lines 1 extend in the lateraldirection (x direction) and are arranged in the longitudinal direction(y direction). The video signal lines 2 extend in the longitudinaldirection and are arranged in the lateral direction. The pixel electrode26 and the TFT are formed in the area surrounded by the scanning lines 1and the video signal lines 2. The TFT in FIG. 2 is a top gate type TFT.

In FIG. 2, the active element (the semiconductor layer) of the TFT isformed by the oxide semiconductor 13. The TFT of the oxide semiconductor13 has a feature of low leak current. Among the oxide semiconductors 13,optically transparent and amorphous materials are called TAOS(Transparent Amorphous Oxide Semiconductor). Examples of TAOS are indiumgallium zinc oxide (IGZO), indium tin zinc oxide (ITZO), zinc oxidenitride (ZnON), and indium gallium oxide (IGO). IGZO is used as theoxide semiconductor 13 in the present embodiments.

In FIG. 2, the gate electrode 15 is formed above the oxide semiconductorfilm 13 via the insulating film. The gate electrode 15 is a branch ofthe scanning line 1. The channel of the TFT is formed immediately belowthe gate electrode 15 via the insulating film in the oxide semiconductorfilm 13. One side, toward the video signal line, of the oxidesemiconductor is the drain, and another side, opposite to the one sideof the oxide semiconductor, is the source.

In FIG. 2, the drain electrode 19 is connected to the oxidesemiconductor film 13 via through hole 17. The drain electrode 19 is abranch of the video signal line 2. The source electrode 20 is connectedto the source of the oxide semiconductor film 13 via through hole 18.The source electrode 20 is connected to the pixel electrode 26 via thethrough hole 22 formed in the organic passivation film and the throughhole 25 formed in the capacitance insulating film.

The pixel electrode 26 is stripe shaped. The common electrode 23 isformed in a plane shape under the pixel electrode 26 via the capacitanceinsulating film. The common electrode 23 is formed all over the areaexcept the through hole 22. When the video signal is applied to thepixel electrode 26, a line of force is generated between the commonelectrode 23 and the pixel electrode 26 through the liquid crystallayer; consequently, liquid crystal molecules are rotated and thus, thetransmittance in each of the pixels is changed.

In the example of FIG. 2, the pixel electrode 26 is constituted by onestripe since the lateral size of the pixel is small as 30 microns. Whenthe lateral size of the pixel becomes bigger, however, the shape of thepixel electrode 26 becomes comb shaped, which has a slit inside. FIG. 2is a structure that is used in so called IPS (In Plane Switching) modeliquid crystal display device.

FIG. 3 is a cross sectional view along the line A-A of FIG. 2. In FIG.3, the light shading film 11 is formed on the TFT substrate 10, which ismade of glass, etc. If a flexible display device is required, the TFTsubstrate 10 is made of resin as polyimide.

The light shading film stops the light from the backlight for the oxidesemiconductor film 13, which is formed above the light shading film;thus, photo current in the oxide semiconductor film 13 is suppressed.The light shading film 11 is made of metal as MoW, etc. The lightshading film 11 can utilize the same material as the gate electrode 15,which is formed later.

The under coat film 12 is formed over the light shading film 11. Theunder coat film 12 prevents the oxide semiconductor film 13 from beingcontaminated by impurities in the TFT substrate 10 made of e.g. glass,as well as insulates the light shading film 11 from the oxidesemiconductor film 13. The under coat film 12 is generally has a twolayer structure of the SiN film and the SiO film; the SiO film contactsthe oxide semiconductor film 13, which is made on the SiO film.

In the meantime, as to be explained in fourth embodiment, some productsuse the light shading film 11 as the bottom gate for the oxidesemiconductor film 13. In this case, the under coat film 12 works as thegate insulating film. The second under coat film can be additionallyused between the light shading film 11 and the substrate 10. In thiscase, the second under coat film, too, may be formed by two layers ofthe SiO film and the SiN film.

In FIG. 3, the oxide semiconductor film 13 is formed on the under coatfilm 12. IGZO is used for the oxide semiconductor film 13. A thicknessof the oxide semiconductor film 13 is 15 to 100 nm. The oxidesemiconductor film 13 is formed by sputtering. The insulating film 14 ofSiO is formed on the oxide semiconductor film 13. Generally, the gateelectrode 15 is formed on the insulating film 14, however, in thepresent invention, aluminum oxide film 30 is formed between the gateelectrode 15 and the insulating film 14. The aluminum oxide film 30 hasa two layer structure.

The interlayer insulating film 16 is formed over the gate electrode 15.The interlayer insulating film 16 can have a one layer structure of SiOfilm or can have a two layer structure of SiO film and SiN film. Whenthe two layer structure is adopted, the lower layer is the SiO film andthe upper layer is the SiN film.

In FIG. 3, the portion of the oxide semiconductor 13 immediately underthe gate electrode 15 is the channel; the left hand side is the drain,the right hand side is the source. The drain and the source are formedby ion implantation of e.g. Ar using the gate electrode 15 as a mask.

In FIG. 3, through holes 17 and 18 are formed in the interlayerinsulating film 16 and the insulating film 14. The drain electrode 19and the drain are connected via the through hole 17 and the sourceelectrode 20 and the source are connected via the through hole 18.

In FIG. 3, the organic passivation film 21 is formed covering the drainelectrode 19 and the source electrode 20. Since the organic passivationfilm 21 has a role as a flattening film, it is formed thick as 2 micronto 4 micron. The common electrode 23 is formed in plane shape on theorganic passivation film 21; the capacitance insulating film 24 isformed on the common electrode 23.

The pixel electrode 26 is formed on the capacitance insulating film 24.An example of a plan view of the pixel electrode 26 is shown in FIG. 2.The pixel capacitance is formed between the common electrode 23 and thepixel electrode 26 through the capacitance insulating film 24.

In FIG. 3, the through hole 22 is formed in the organic passivation film21 and the through hole 25 is formed in the capacitance insulating film24 to connect the pixel electrode 26 to the source electrode 20. Thealignment film 27 is formed on the pixel electrode 26. The alignmentfilm 27 is for initial alignment of the liquid crystal molecules 301;the alignment is conducted through the rubbing method or the opticalalignment method using ultra violet ray. In the case of IPS mode, theoptical alignment method is suitable. When video signals are applied tothe pixel electrode 26, a line of force as depicted by arrows in FIG. 3is generated between the pixel electrode 26 and the common electrode 23and liquid crystal molecules 301 are rotated; consequently, thetransmittance of the liquid crystal layer 300 is controlled.

In FIG. 3, the counter substrate 200 is set opposing to the TFTsubstrate 10 sandwiching the liquid crystal layer 300. Generally, thecounter substrate 200 is formed by glass; the resin like polyimide isused for the counter substrate 200 when a flexible display device isrequired. The color filter 201 and the black matrix 202 are formed on aninner side of the counter substrate 200. The over coat film 203 isformed covering the color filter 201 and the black matrix 202. Thealignment film 204 is formed on the over coat film 203. The alignmentprocess for the alignment film 204 is the same as that of the alignmentfilm 32 on the TFT substrate 10.

FIG. 4 is a cross sectional view of the TFT and its vicinity to show thefeature of this embodiment. In FIG. 4, the light shading film 11 isformed on the substrate 10; the under coat film 12 is formed on thelight shading film 11; and the oxide semiconductor film 13 is formed onthe under coat film 12. The oxide semiconductor film 13 is formed byIGZO. The insulating film 14 is formed covering the oxide semiconductorfilm 13. The insulating film 14 is formed by oxide rich SiO to maintainthe amount of oxygen in the oxide semiconductor film 13.

The gate electrode 15 is formed over the insulating film 14, however,the aluminum oxide film 30, which has a two layer structure of the firstaluminum oxide film 31 and the second aluminum oxide film 32, is formedbetween the gate electrode 15 and the insulating film 14. As will beexplained later, the first aluminum oxide film 31 and the secondaluminum oxide film 32 are formed to maintain the amount of oxygen inthe oxide semiconductor film 13.

The gate electrode 15 is made of either Titanium (Ti) or Aluminum (Al).In this specification, the aluminum alloy that contains aluminum as amajor substance like AlSi and so forth is also expressed as aluminum.The interlayer insulating film 16 is formed covering the gate electrode15. The through holes 17 and 18 are formed in the interlayer insulatingfilm 16 and the insulating film 14 to connect the oxide semiconductorfilm 13 and the drain electrode 19 or to connect the oxide semiconductorfilm 13 and the source electrode 20.

FIG. 5 is an enlarged cross sectional view of the gate electrode 15 andits vicinity. The feature of this embodiment is to form the firstaluminum oxide film 31 and the second aluminum oxide film 32 between thegate electrode 15 and the insulating film 14. In FIG. 5, at the outset,the first aluminum oxide film 31 is formed in a thickness of 2 nm bysputtering. The first aluminum oxide film 31 is made oxygen rich filmformed by oxygen reactive mode sputtering, namely, sputtering in theoxygen environment. Since the oxygen reactive mode sputtering gives somedamage to the insulating film 14, a thickness of the first aluminumoxide film 31 is limited to between 2 to 5 nm, preferably 2 nm. Sincedeposition rate in the oxygen reactive mode sputtering is low, it takesapproximately 1 minute to deposit the first aluminum oxide film in 2 nm.

Subsequently, the second aluminum film 32 is formed by transition modesputtering, decreasing the amount of oxygen in the sputtering chamber. Athickness of the second aluminum oxide film 32 is made 5 to 10 nm. Inthe transition mode sputtering, the deposition rate is higher than thatin the oxygen reactive mode sputtering. The aluminum oxide film formedby transition mode sputtering has lower oxygen content than that of thealuminum oxide film formed by oxygen reactive mode sputtering. In otherwords, when aluminum oxide is expressed by AlyOx, the value x/y isbigger in the aluminum oxide formed by the oxygen reactive mode than inthe aluminum oxide formed by the transition mode.

FIG. 6 is a cross sectional view in the sputtering equipment. In FIG. 6,the cathode 101 and the anode 100 oppose to each other withpredetermined distance. The aluminum target 102 is set on the cathode101. The substrate 10, on which the sputtering film 30 is deposited, isset on the anode 100.

The plasma 103 for sputtering is formed by adding predetermined gassesand applying a voltage. The gas is oxygen (O₂) added Argon (Ar); thesputtering mode is determined by the amount of oxygen.

FIG. 7 is a graph showing the sputtering modes. In FIG. 7, the abscissais the amount of flow of oxygen. The sputtering mode is changed from themetal mode, to the transition mode, and to the oxygen reactive modeaccording to the amount of flow of oxygen. The amount of flow of Argonis constant in all the modes. The film formed under the oxygen reactivemode sputtering contains a large amount of oxygen; the film formed underthe metal mode sputtering becomes an aluminum film or a film thatcontains extremely low amount of oxygen; the film formed under thetransition mode sputtering contains an amount of oxygen in between. Inother words, when aluminum oxide is expressed by AlyOx, the value x/ybecomes bigger according the amount of flow of oxygen in the sputteringprocess.

The ordinate of FIG. 7 is a discharge voltage. The discharge voltage isproportional to the deposition rate. Namely, the deposition rate is highin the metal mode and in the transition mode; the deposition rate is lowin the oxygen reactive mode.

FIGS. 8-12 are cross sectional views of the process that realizes thestructure of FIG. 5. FIG. 8 is a cross sectional view that shows thefirst aluminum oxide film 31 is formed on the insulating film 14 byoxygen reactive mode sputtering. A thickness of the first aluminum film31 is e.g. 2 nm. Since the deposition rate in the oxygen reactive modeis low, approximately 1 minute sputtering is necessary for 2 nmdeposition.

During the oxygen reactive mode sputtering, an excessive amount ofoxygen can be implanted in the insulating film 14, which is made of SiO,there is a chance that the insulating film 14 gets damage; consequentlythe reliability of the TFT could decrease. Therefore, a thickness of thefirst aluminum oxide film 31, which is formed by oxygen reactive modesputtering, is preferably 5 nm or less, and most preferably 2 nm. Inaddition, since particles are tend to be generated in the oxygenreactive mode sputtering, it is preferable to keep the thickness of thefirst aluminum oxide film 31 thin from this aspect too. By the way, eventhe thickness of the first aluminum oxide film 31 is as thin as 2 nm,the thickness can be measured by TEM (Transmission Electron Microscopy).

FIG. 9 is a cross sectional view that the second aluminum oxide film 32is formed by sputtering on the first aluminum oxide film 31. The secondaluminum oxide film 32 is formed by the transition mode sputtering.Damage to the insulating film 14 due to oxygen is lower in thetransition mode sputtering compared with that in the oxygen reactivemode sputtering. The deposition rate in the transition mode is high anda chance of generation of particles during the transition modesputtering is low.

However, since the membrane stress of the aluminum oxide film formed bytransition mode sputtering tends to be high, peeling off of the filmtends to occur when the film is made thick. Therefore, a thickness ofthe second aluminum oxide film 32 is preferably 5 to 15 nm.

FIG. 10 is a cross sectional view that the metal, which is to be thegate electrode 15, is formed by sputtering on the second aluminum oxidefilm 32. The metal for the gate electrode 15 is e.g. Ti, Al, MoW or soforth. The metal, especially Ti and Al, easily absorb oxygen. Therefore,the metal extracts oxygen from the oxide semiconductor film 13 throughthe insulating film 14. In this embodiment, however, the first aluminumoxide film 31 and the second aluminum oxide film 32 are formedbeforehand, thus, those films can be block films to prevent extractionof oxygen from the oxide semiconductor film 13.

FIG. 11 is a cross sectional view that the gate electrode 15, the secondaluminum oxide film 32 and the first aluminum oxide film 31 arepatterned. FIG. 12 is a cross sectional view wherein after the gateelectrode 15 is patterned, ion implantation is conducted using the gateelectrode 15 as the mask to implant e.g. Ar in the oxide semiconductorfilm 13, except the portion under the gate electrode 15, to giveconductance to the oxide semiconductor film 13.

The first aluminum oxide film 31 and the second aluminum oxide film 32exist under the gate electrode 15. The second aluminum oxide film 32prevents the extraction of the oxygen by the gate electrode 15 from theoxide semiconductor film 13; the first aluminum oxide film 31, which isoxygen rich film, supplies oxygen to the oxide semiconductor film 13through the insulating film 14.

As described above, TFTs of stable characteristics having the oxidesemiconductor film 13 as the active layer can be realized according tothe first embodiment.

Second Embodiment

FIG. 13 is a cross sectional view according to second embodiment. FIG.13 differs from FIG. 4 in that the insulating film 14 is formed onlyunder the gate electrode 15. Namely, the oxide rich insulating film 14is formed only under the gate electrode 15. In the oxide semiconductorfilm 13, it is only channel region in which supplying of the oxygen isnecessary or preventing extraction of oxygen is necessary.

The region of the oxide semiconductor film 13 other than the channel,namely the drain region and the source region, should be conductive;therefore, it is preferable that oxygen does not exist in this region.In the structure of FIG. 13, since oxygen rich insulating film 14 doesnot exist on the drain and the source, unnecessary oxygen is notsupplied to the oxide semiconductor film 13.

In FIG. 13, the interlayer insulating film 16 contacts the drain and thesource of the oxide semiconductor film 13. The interlayer insulatingfilm 16 can be made of SiO film, the SiO film that constitutes theinterlayer insulating film 16 contains less oxygen than the insulatingfilm 14 does; therefore, supply of oxygen to the drain and the source ofthe oxide semiconductor film 13 from the interlayer insulating film 16is limited.

As described above, in the structure of FIG. 13, the first aluminumoxide 31 can supply oxygen to the channel of the oxide semiconductorfilm 13 for which supplying of the oxygen is necessary; the secondaluminum oxide film 32 prevent extraction of the oxygen by the gateelectrode 15 from the oxide semiconductor film 13 through the insulatingfilm 14. In addition to that, the insulating film 14 does notexcessively supply oxygen to the drain and the source; thus, TFTs ofoxide semiconductor having stable characteristics can be manufactured.

Third Embodiment

FIG. 14 is a cross sectional view that shows third embodiment. FIG. 14differs from FIG. 13 of second embodiment in that the second aluminumoxide film 32 is formed by oxidation in the annealing process, not thefilm formed by sputtering. The process is as follows: the gate electrode15 is formed by aluminum; after the gate electrode 15 is patterned, thesurface of the gate electrode 15 is oxidized in the annealing process toform the second aluminum oxide film 32.

After the gate electrode 15 is patterned, the drain and the source ofthe oxide semiconductor 13 is given conductivity by driving in Ar and soforth by ion implantation; subsequently the oxide semiconductor film 13is necessary to be activated by annealing. In FIG. 14, the aluminumoxide formed on the surface of the aluminum during the annealing is usedas the second aluminum oxide film 32. This structure can also performthe same function of the subject invention as explained in firstembodiment and second embodiment.

FIGS. 15-18 are cross sectional views of the process to constitute thestructure of FIG. 14. FIG. 15 is a cross sectional view that the firstaluminum oxide film 31 is formed by oxygen reactive mode sputtering onthe insulating film 14. Manufacturing method and thickness etc. of thefirst aluminum oxide film 31 is the same as explained in firstembodiment.

FIG. 16 shows the aluminum film, which constitutes gate electrode 15, isformed by sputtering etc. over the first aluminum oxide film 31. FIG. 17is a cross sectional view that the gate electrode 15, the first aluminumoxide film 31, the insulating film 14 are patterned. In FIG. 17, thegate electrode 15 made of aluminum is formed directly on the firstaluminum oxide film 31. In FIG. 17, after the gate electrode 15 ispatterned, the drain and the source of the oxide semiconductor film 13are given conductivity except the portion under the gate electrode 15 ofthe oxide semiconductor film 13 by performing ion implantation using thegate electrode 15 as the mask.

FIG. 18 is a cross sectional view, in which the gate electrode 15 andthe oxide semiconductor film 13 etc. are covered by the interlayerinsulating film 16; then the aluminum oxide film is being generated onthe surface of the gate electrode 15 by annealing at 250 centigrade to350 centigrade. In FIG. 18, a thickness of the second aluminum film 32,formed between the gate electrode 15 and the first aluminum oxide film31 by annealing, is approximately 2 nm.

The oxygen for the formation of the second aluminum oxide 32 is suppliedfrom the first aluminum oxide film 31. Since the first aluminum oxidefilm 31 is an oxygen rich film, it can supply oxygen to the gateelectrode 15 during the annealing for the formation of the secondaluminum oxide film 32. FIG. 19 is a graph that shows the amount ofreleased oxygen when the aluminum oxide film is heated; the amount ofoxygen released from the aluminum oxide is detected by TDS (ThermalDesorption Spectrometry).

In FIG. 19, the abscissa is the temperature of the substrate; theordinate is the amount of oxygen released from the aluminum oxide,namely, intensity of O₂ (atomic weight is 32). In FIG. 19, desorption ofoxygen increases in proportion to the temperature in the region of 100centigrade to approximately 350 centigrade. FIG. 19 shows a relativevalue; however, when the amount of oxygen in the aluminum oxide film isbigger, the amount of desorption of oxygen increases.

Back to FIG. 18, the second aluminum oxide film 32 is formed not onlybetween the gate electrode 15 and the first aluminum oxide 31 but alsoformed between the gate electrode 15 and the interlayer insulating film16. In this case, the oxygen is supplied from the surrounding interlayerinsulating film 16, which is made of SiO. Therefore, the second aluminumoxide film 32 covers all around the gate electrode 15 made of metal,thus, absorption of oxygen by the gate electrode 15 is suppressed.

The oxygen concentration in the second aluminum oxide film 32 that isformed by annealing is lower than that of the first aluminum oxide film31. Therefore, the distribution of oxygen in the aluminum oxide film 30is the same as that in first embodiment and second embodiment.

By the way, when the aluminum oxide film is formed on the surface of thegate electrode by annealing, there could occur a problem of anelectrical connection between the gate electrode 15 and the upperelectrode 28 or other wirings as depicted in FIG. 20. Unlike FIG. 2, thegate electrode 15 in FIG. 20 is supplied with gate voltage from theupper electrode 28 via through hole 161 in the insulating film 16. InFIG. 20, the aluminum oxide film 32, which is an insulator, existsbetween the upper electrode 28 and the gate electrode 15.

FIG. 21 is a graph that shows a contact resistance when the substrate isannealed, in the structure shown in FIG. 20. In FIG. 21, the abscissa isannealing temperature; the ordinate is a contact resistance between thegate electrode 15 and the upper electrode 28 in the contact hole 161.

As depicted in FIG. 21, when annealing temperature is 250 centigrade ormore, the contact resistance between the electrode 28 and the gateelectrode 15 drastically decreases and approaches to an insignificantvalue that is essentially no problem. The possible reason is that: sincethe aluminum oxide film formed by annealing is in a thickness ofapproximately 2 nm, the metal, constituting the gate electrode 15, orthe metal, constituting electrode 28, diffuses into the aluminum oxidefilm 32 when annealing is conducted with high temperature; even theamount of diffusion may be small, electrical connection in the throughhole can be achieved.

In general, in order to decrease the contact resistance in the throughhole, the through hole is cleansed with hydro fluoride (HF). In FIG. 21,circles are without HF cleansing; rhombuses are with HF cleansing. FIG.21, however, shows that raising the temperature of annealing is moreeffective to decrease the contact resistance in the through hole thancleansing the through hole with HF. As described above, if a problem ofcontact resistance due to the aluminum oxide formed by annealing occurs,the following process is effective: forming the through hole forelectrical connection, forming the electrode for connection in thethrough hole, after that, the annealing is applied to decrease thecontact resistance in the through hole.

Fourth Embodiment

In first embodiment to third embodiment, the invention is explained inthe case of top gate type TFT, in which the gate electrode 15 is setabove the oxide semiconductor film 13. The present invention, however,is applicable to the bottom gate type TFT, in which the gate electrode15 is set beneath the oxide semiconductor film 13, too. Further, thepresent invention is applicable to the dual gate type TFT, in which thegate electrode 15 exists both above and beneath the oxide semiconductorfilm 13.

FIG. 22 is a cross sectional view of the pixel area in the liquidcrystal display device which uses the dual gate type TFT, in which thegate electrode 15 is above the oxide semiconductor film 13 and the gateelectrode 41 is beneath the oxide semiconductor film 13. FIG. 22 differsfrom FIG. 4 in first embodiment in that the bottom gate 41 is setbeneath the oxide semiconductor film 13.

In FIG. 22, the second under coat film 40 is formed on the substrate 10,which is made of glass and so forth. The second under coat film 40 has aplural layer structure formed by SiN film and SiO film. The bottom gateelectrode (the second gate electrode) 41 is formed on the second undercoat film 40. The second gate electrode 41 has also a role of lightshading film for the oxide semiconductor film 13.

The aluminum oxide film 50 is formed on the second gate electrode 41.This aluminum oxide film 50 has a same role as the aluminum oxide film30 in the top gate. The aluminum oxide film 50 also has a two layerstructure as with the aluminum oxide film 30, however, the processingorder of the first aluminum oxide 51 and the second aluminum oxide 52 isdifferent.

After that, the second gate insulating film 42 is formed. The secondgate insulating film 42 is expressed as the under coat film in firstembodiment. In this embodiment, the second gate insulating film 42 ismade of SiO film. The second gate insulating film 42 can have a twolayer structure of SiO film and SiN film. After that the oxidesemiconductor film 13 is formed. The structure above the oxidesemiconductor film 13 is the same as FIG. 3.

FIG. 23 is an enlarged cross sectional view of the second gate electrode41 and its vicinity. In FIG. 23, the second gate electrode 41 made ofe.g. aluminum is formed on the under coat film 40, which is formed onthe substrate 10. The aluminum oxide film 50 is formed on the secondgate electrode 41; the lower layer, which is formed first, is the secondaluminum oxide film 52, formed in a thickness of 5 to 10 nm bytransition mode sputtering. The oxygen rich first aluminum oxide film 51is formed in a thickness of 2 to 5 nm, preferably 2 nm, by oxygenreactive mode sputtering on the second aluminum oxide film 52.

The manufacturing method is as follows. The metal for the second gateelectrode 41 is formed by sputtering on the second under coat film 40;the second aluminum oxide film 52 is formed by transition modesputtering on the second gate electrode 41; the first aluminum oxidefilm 51 is formed by oxygen reactive mode sputtering on the secondaluminum oxide film 52. After that the resist is coated and patterned byphoto lithography, subsequently, the first aluminum oxide film 51, thesecond aluminum oxide film 52 and the second gate electrode 41 arepatterned by dry etching or wet etching.

After that, the second gate insulating film 42 is formed covering thefirst aluminum oxide film 51. The second gate insulating film 42 is madeof SiO. The second gate insulating film 42 can be a two layer structureof SiO film and SiN film; in this case, the SiO film is set to contactthe oxide semiconductor film 13. The oxide semiconductor film 13 isformed by sputtering and patterned. The process after that is the sameas first embodiment.

The functions of the first aluminum oxide film 51 and the secondaluminum oxide film 52 in FIGS. 22 and 23 are the same as the firstaluminum oxide film 31 and the second aluminum oxide film 32 explainedin first embodiment. As described above, the present invention can beapplied to the TFTs of the bottom gate type and the dual gate type.

In first embodiment to fourth embodiment, it was explained that the twolayers of aluminum films were formed between the insulating film and thegate electrode. However, even two layer structure is formed initially, aboundary between the first aluminum oxide film 31 and the secondaluminum oxide film 32 may become obscure due to annealing process andso forth, which is applied later. Even this case, concentration ofoxygen in the aluminum oxide film is larger at the side of theinsulating layer than at the side of the gate electrode.

In first embodiment to fourth embodiment, it was explained that the gateelectrode was made of metal, its major substance is aluminum. Thepresent invention is, however, applicable to the case where the gateelectrode is made of other metals that can form metal oxide at thesurface, like Ti (Titanium).

In the above explanation, the present invention was explained in thecase of IPS mode liquid crystal display device. The present inventionis, however, applicable to other types of liquid crystal displaydevices. The organic EL display device also uses the oxide semiconductorTFTs. The cross sectional structure of the organic EL display device isbasically the same as FIG. 3 up to formation of the organic passivationfilm 21. Therefore, the present invention explained above can be appliedto the organic EL display device.

What is claimed is:
 1. A display device including a substrate havingthin film transistors (TFT) comprising: the TFT including an oxidesemiconductor film, a gate electrode and an insulating film formedbetween the oxide semiconductor film and the gate electrode, wherein afirst aluminum oxide film and a second aluminum oxide film, which isformed on the first aluminum oxide film, are formed between theinsulating film and the gate electrode, and an oxygen concentration inthe first aluminum oxide film is larger than an oxygen concentration inthe second aluminum oxide film.
 2. The display device according to claim1, wherein a thickness of the second aluminum oxide film is thicker thana thickness of the first aluminum oxide film.
 3. The display deviceaccording to claim 1, wherein a thickness of the first aluminum oxidefilm is 2-5 nm.
 4. The display device according to claim 1, wherein athickness of the second aluminum oxide film is 5-10 nm.
 5. The displaydevice according to claim 1, wherein the gate electrode is made of ametal, which has aluminum as a major substance.
 6. The display deviceaccording to claim 1, wherein the insulating film is made of siliconoxide.
 7. The display device according to claim 1, wherein the TFT is atop gate type TFT.
 8. The display device according to claim 1, whereinthe insulating film is formed between the gate electrode and the oxidesemiconductor film, and the insulating film is not formed on a drain ora source of the oxide semiconductor film.
 9. A display device includinga substrate having thin film transistors (TFT) comprising: the TFTincluding an oxide semiconductor film, a gate electrode and aninsulating film formed between the oxide semiconductor film and the gateelectrode, wherein an aluminum oxide film is formed between theinsulating film and the gate electrode, and an oxygen concentration inthe aluminum oxide film is larger at a side of the insulating film thanan oxygen concentration at a side of the gate electrode.
 10. The displaydevice according to claim 9, wherein the gate electrode is made of ametal, which has aluminum as a major substance.
 11. A manufacturingmethod of a display device including a substrate in which the thin filmtransistors (TFT) are formed, the TFT including an oxide semiconductorfilm, a gate electrode, and an insulating film formed between the oxidesemiconductor film and the gate electrode, the manufacturing methodcomprising: forming a first aluminum oxide film on the insulating film,forming a second aluminum oxide film on the first aluminum oxide film,an oxygen concentration of the second aluminum oxide film is smallerthan an oxygen concentration of the first aluminum oxide film, forming ametal for the gate electrode, and patterning the gate metal, the secondaluminum oxide film and the first aluminum oxide film.
 12. Themanufacturing method according to claim 11, wherein a thickness of thesecond aluminum oxide film is thicker than a thickness of the firstaluminum oxide film.
 13. The manufacturing method according to claim 11,wherein the first aluminum oxide film is formed by oxygen reactive modesputtering.
 14. The manufacturing method according to claim 11, whereinthe second aluminum oxide film is formed by transition mode sputtering.15. The manufacturing method according to claim 11, wherein the gateelectrode is made of a metal, which has aluminum as a major substance.16. The manufacturing method according to claim 11, wherein theinsulating film is made of silicon oxide.
 17. A manufacturing method ofa display device including a substrate in which the thin filmtransistors (TFT) are formed, the TFT including an oxide semiconductorfilm, a gate electrode and an insulating film formed between the oxidesemiconductor film and the gate electrode, the manufacturing methodcomprising: forming a first aluminum oxide film on the insulating film,forming a gate electrode, patterning the gate electrode, forming asecond aluminum oxide film between the gate electrode and the firstaluminum oxide film by annealing the gate electrode.
 18. Themanufacturing method according to claim 17, wherein the gate electrodeis made of a metal, which has aluminum as a major substance.
 19. Themanufacturing method according to claim 17, wherein the first aluminumoxide film is formed by oxygen reactive mode sputtering.
 20. Themanufacturing method according to claim 17, wherein the first aluminumoxide film is formed so that an oxygen concentration of the firstaluminum oxide film is larger than an oxygen concentration of the secondaluminum oxide film.